This invention relates to fault-tolerant computer systems. It is well known that fault tolerance can be achieved in a computer system by replicating components of the system. For example, two processing units may be provided, one of which acts as the active unit and the other of which acts as a stand-by. When a fault is detected in the active unit, from which it cannot recover, the stand-by is switched in as the active unit, allowing system operation to continue. Similarly, it is known to replicate data storage units using disk mirroring systems.
A problem which arises in such a system is how to locate faults precisely, to enable more appropriate recovery action to be taken. The object of the invention is to provide a novel way of solving this problem.